Master or Mr for my 7 month olds first passport

Master or Mr for my 7 month olds first passport

Hi,

Just wondered whether I should complete my sons first passport application, using the title Mr or Master?? I know it doesn't show on the actual passport, but for completing the application purposes im unsure which to use.
Obviously his photo has to be countersigned, using the exact title I give on the form, so want to make sure its correct!!